1. Field of the Invention
The present invention relates to a dual phase high strength cold-rolled steel plate, and more particularly to a cold-rolled steel plate which shows excellent formability in press-forming or other shaping operations and has a sufficient strength after a forming operation, along with a good galvanizing property.
2. Description of the Prior Art
For filling the recent strong demand for reductions of body weights of motor vehicles, there has been a tendency of using sheets of high tension steel of smaller thicknesses. The steel sheets are also required to be satisfactory in formability and other properties when worked in a galvanized form. Steel sheets have a general tendency in their properties that their yield points are elevated with increases in tensile strength. That is to say, a higher strength is reflected by lower forming characteristics, giving rise to various problems such as spring-back, galling during press-forming operations. In this regard, there are many reports on the results of attempts which have thus far been made in various aspects to lower the yield point of high strength steel, for example, a composite structure (or dual phase) steel sheet which is produced by continuous annealing technology has a low yield ratio and is free of elongation at yield point, receiving wide attention and consideration as a sheet steel for motor vehicles.
In order to obtain the composite structure (normally consisting of a ferrite phase surrounded by uniformly dispersed low-temperature transformation products such as martensite and bainite), it is generally required to retain a soaking period longer than one minute unlike the nomenclature "continuous annealing". It is not known to form a composite structure during annealing of a shorter time period. The annealing time in an ordinary continuous molten zinc galvanizing line, however, is 20 to 30 seconds at longest. It is therefore very practical if the formation of the composite structure is completed within such a short time period, but the present inventors do not known nor are aware of any report which give discussions on this point.